The AC current transformer has been well-known for many decades, and is used where substantially alternating currents are to be measured. Current transformers are often used when galvanic isolation is required between the circuit being measured and apparatus responsive to current in the measured circuit. However, when direct current flows in the circuit being measured, the AC current transformer is, by itself, unable to transform any DC portion of its input current to responsive apparatus. Moreover, if sufficient direct current flows in a current transformer, core saturation is likely, preventing transformation of either alternating or direct current.
The measurement of DC currents without galvanic isolation is well-known, and is not relevant to this invention. This invention is one form of a type of apparatus known as direct-current current transformers, or DCCT's. Many prior art DCCT's supplement an AC current transformer to provide DC response. One common method is exemplified by the AC/DC current probe of U.S. Pat. No. 7,309,980, wherein a Hall-effect transducer provides low frequency transformation. Such devices have proven useful, but are subject to thermal drift, and to stray magnetic fields and, may require de-gaussing. Another method is alternately to saturate and de-saturate a current transformer core to convert the DC flux therein into a transformable AC signal. This method is exemplified by the current probe of U.S. Pat. No. 6,885,183, which also exemplifies another much-used method, to wit, that of using a DC feedback winding to cancel the DC flux in the transformer core. Generally, the methods using DC feedback are either complex or have bandwidth limited to a few KHz. Numerous other methods, some very complex, have been employed to provide a direct-current current transformer, or DCCT.